Aircraft typically include a flight control system for directional and attitude control of the aircraft in response to commands from a flight crew or an autopilot. A flight control system may include a plurality of movable flight control surfaces such as ailerons on the wings for roll control, elevators on the horizontal tail of the empennage for pitch control, a rudder on the vertical tail of the empennage for yaw control, and other movable control surfaces. Movement of a flight control surface is typically effected by one or more actuators mechanically coupled between a support structure (e.g., a wing spar) and the flight control surface (e.g., an aileron). In many aircraft, the actuators for flight control surfaces are linear hydraulic actuators driven by one or more hydraulic systems which typically operate at a fixed working pressure.
One of the challenges facing aircraft designers is preventing the occurrence of flutter of the flight control surfaces during flight. Control surface flutter may be described as unstable aerodynamically-induced oscillations of the flight control surface, and may occur in flight control systems where the operating bandwidth of the flight control system overlaps the resonant frequency of the flight control surface. Unless damped, the oscillations may rapidly increase in amplitude with the potential for undesirable results, including exceeding the strength capability of the mounting system of the flight control surface and the actuator. Contributing to the potential for control surface flutter is elasticity in the flight control system. For example, hydraulic actuators may exhibit a linear spring response under load due to compressibility of the hydraulic fluid. The compressibility of the hydraulic fluid may be characterized by the cross-sectional area of the actuator piston, the volume of the hydraulic fluid, and the effective bulk modulus of elasticity of the hydraulic fluid.
One method of addressing control surface flutter involves designing the flight control system such that the operating bandwidth does not overlap the resonant frequency of the flight control surface, and may include limiting the inertia of the load on the actuator and/or increasing the piston cross-sectional area as a means to react the inertia load. Unfortunately, the above known methods result in an actuator system that is sized not to provide the actuator with static load-carrying capability, but rather to provide the actuator with the ability to react larger inertia as a means to avoid resonance in the operating bandwidth. As may be appreciated, limiting control surface inertia corresponds to a decrease in control surface area. A decrease in the surface area of higher inertia control surfaces of an aircraft empennage may reduce the attitude controllability of the aircraft. As may be appreciated, an increase in the piston cross-sectional area of an actuator corresponds to an increase in the size and weight of the hydraulic system components including the size and weight of the actuators, tubing, reservoirs, and other components. The increased size of the actuators may protrude further outside of the outer mold line of the aerodynamic surfaces resulting in an increase in aerodynamic drag of an aircraft. The reduced attitude controllability, increased weight of the hydraulic system, and increased aerodynamic drag may reduce safety, fuel efficiency, range, and/or payload capacity of the aircraft.
As can be seen, there exists a need in the art for a system and method for allowing the operating bandwidth of an actuator to match or encompass the resonant frequency of a movable device without oscillatory response.
In addition, flutter suppression is a known challenge for high-pressure, hydraulic, flight-control actuation. High pressure hydraulics systems face an upper limit due to aero-servo-elasticity which drives a lower limit on actuator linear stiffness. That lower limit depends on the kinematics and inertia of the flight control surface.
Known flight control systems and method for addressing flutter suppression are primarily focused on increasing linear stiffness by increasing actuator piston diameter, which may cause increased flight control system and aircraft size, weight, and power. Increased flight control system and aircraft size, weight, and power may result in increased flight fuel costs. Other known flight control systems and methods for addressing flutter suppression attempt to enhance the active control system performance by increasing the servo bandwidth to operate in the high dynamic resonant frequency range of the actuator and valve. However, such known flight control systems and methods involve the used of active control elements, such as the actuator and valve size or diameter, rather than a passive means to change the dynamics of the flight control system. The use of such active control elements may overly complicate the control elements and be less space efficient.
As can be seen, there exists a need in the art for an assembly and method to address flutter suppression and flutter critical control surface applications on aircraft, to dampen movement of flight control surfaces, and to optimize a flight control system design in terms of space efficiency and changing the dynamic characteristics of the hardware under control rather than complicating the flight control system elements themselves.